A cladding-pumped fiber laser typically comprises a single mode fiber core disposed within a relatively large multi-mode cladding. This inner cladding is surrounded by a second, outer cladding to prevent radiation from propagating out of the inner cladding. The fiber is positioned between two mirrors or gratings to define a laser cavity. Light from a pump laser, such as a laser diode, is injected into the end of the cladding. Typically, the length of the fiber is sufficient for absorbing most of the pure radiation, which propagates in the core, by the active laser species located in the core. This is advantageous since radiation can be coupled into the multi-mode cladding without the high tolerances typically required for coupling light directly into a single mode core. The core is typically doped with rare-earth ions, which are the active lasing species. The ions absorb photons delivered by the pump laser. Photons are then emitted by the ions at a wavelength characteristic of the particular dopant species.
Of particular importance are high power, multi-watt fiber lasers that operate at shorter wavelength, i.e. at less than 1065 nm. The radiation is used to pump Erbium/Ytterbium (Er/Yb) fiber amplifiers operating in the 1.55 .mu.m. Cladding-pumped Yb fiber lasers that produce multi-watt outputs at 1065 nm have been demonstrated with special high-brightness diode laser sources. However, efficient operation of such lasers at less than 1065 nm is difficult to achieve with commercially available low brightness pump sources, such as diode-coupled fiber bundles.
One problem with the previously known cladding-pumped Yb fiber lasers is that parasitic lasing and amplified spontaneous emission (ASE) occur at a longer wavelength than the design wavelength. When parasitic operations occur, lasing at the design wavelength cannot be achieved, unless the device efficiency is sacrificed by reducing the length of the fiber laser. These limitations become more significant in high power multi-watt applications.
Remedies against parasitic lasing include incorporating highly-selective Bragg gratings as resonator reflectors and angle-cleaved fiber ends for reducing the feedback at the parasitic wavelength. In addition, the fiber laser length may be reduced until parasitic lasing gives way to operation at the desired wavelength. Shortening the length of the fiber, however, is highly undesirable since it reduces the amount of absorbed pump power. This reduces the overall efficiency of the system.
Thus, there is a need for an improved cladding-pumped laser structure that operates at lower wavelengths and reduces parasitic lasing.